The nuclear envelope (NE) in eukaryotic cells separates nuclear and cytoplasmic compartments. The NE is comprised of an inner nuclear membrane (INM) and an outer nuclear membrane (ONM), which are lined with numerous proteins including those of the LInker-complex between the Nucleoskeleton and the Cytoskeleton (LINC). Mutations in several LINC members cause cardiomyopathies. Luma is a newly discovered member of the LINC complex in the INM. A single amino acid substitution of serine 358 to leucine (S358L) in Luma causes an autosomal-dominant, fully penetrant, cardiomyopathy. Luma interacts with other members of the LINC complex that reside in the INM, including Emerin, Lamins, and SUN2. Luma may play a role in Emerin localization, as HeLa cells depleted of Luma by RNA interference exhibit altered localization of Emerin and a reduction in Emerin levels. Another partner of Lama, LaminA/C directly interacts with chromosomes, and mutations in Lamin result in aberrant chromosome positioning and correlated changes in gene expression. The foregoing results have led us to the hypothesis that Luma plays a key role in cardiomyocyte nucleoskeletal and cytoskeletal integrity, chromosome positioning, and gene expression, and that the S358L mutation in Luma impairs specific aspects of Luma function to lead to cardiomyopathy. Accordingly, our Specific Aims are: 1) To characterize roles of Luma in developing and adult cardiomyocytes by elucidating Luma's subcellular localization and interaction partners, and by performing detailed histological and physiological analyses of cardiac specific Luma knockout mouse models utilizing a floxed allele of Luma. 2) To elucidate molecular mechanisms underlying cardiomyopathy consequent to the S358L mutation in Luma by detailed histological and physiological analyses of a Luma S358L knock-in mouse model. 3) To investigate mechanisms by which the Luma S358L mutation impacts human cardiomyocyte function, utilizing human induced pluripotent stem cell (iPSC) and human embryonic stem cell (hESC)-derived Luma- mutant cardiomyocytes.